This invention relates generally to multilayer printed circuit boards that are mechanically stable under varying temperature conditions and, more particularly, to mechanicaly-stable multilayer circuit boards that are useful for very high frequency electronics (50-400 MHz).
Multilayer printed circuit boards are generally used for interconnecting large numbers of integrated-circuit (IC) chips. The IC chips are usually hermetically sealed in individual chip carrirrs fabricated from a ceramic material, such as aluminum oxide. Bonded leads are brought out from each IC chip to the edges of the chip carrier, and the chip carrier is then soldered, by its leads, directly to the circuit board. The principal advantages of this structure are significantly higher circuit densities, improved speed and impedance characteristics, and substantially reduced packaging costs.
However, a major drawback of using chip carriers is that the coefficient of thermal expansion of aluminum oxide is approximately one-half the coefficient of thermal expansion of the glass/epoxy laminates typically used in the manufacture of circuit boards. When the resulting structure is exposed to any significant range of temperatures, the thermal cycling of the structure can crack soldered joints and render the circuit inoperative. One solution to this problem is to use an intermediate member between the chip carrier and the circuit board. The intermediate member is sometimes referred to as a baby board and the circuit board as a mother board. The intermediate member may also take the form of a hybrid package on which the chip carrier is mounted. Another technique is to use a compliant lead structure between the chip carrier and the circuit board, although this increases the packaging costs and results in excessively long lead lengths. Accordingly, a circuit board should have a coefficient of thermal expansion that closely matches that of the chip carriers mounted on the board.
Another difficulty that has arisen as large numbers of components are mounted on circuit boards is that the heat produced by the components must be dissipated in some manner, whether by conduction through the circuit board or by radiative, convective, or forced-air cooling. Since the principal materials used in circuit boards are insulators, the boards themselves have traditionally played no significant role in dissipating heat from the components that they support.
A third factor in the design of circuit boards is that they should be fabricated from a material with a relatively low dielectric constant, to enhance the board's ability to propagate signals over relatively long distances. Some materials, such as polytetrafluoroethylene (PTFE), have good dielectric properties but an undesirably high coefficient of thermal expansion. Kevlar (trademark of E.I. du Pont de Nemours & Co., Inc.) has a negative coefficient of thermal expansion and may be used to reduce the average coefficient of thermal expansion of a multilayer circuit board. However, Kevlar is a poor thermal conductor and, therefore, does nothing to enhance the thermal conduction properties of the board.
The parent application of this continuation-in-part application discloses a multilayer printed circuit board having low dielectric properties, a low or negative thermal coefficient of thermal expansion, and good thermal conduction properties to enhance heat conduction from the components mounted on the board. However, this multilayer printed circuit board is not suitable for very high frequency electronics. The conventional copper traces that are used to establish connections between the electrical components mounted on this circuit board can slow down very high frequency signals and cause line ringing and crosstalk between the signals. Accordingly, there is a need for a multilayer printed circuit board that not only has low dielectric properties, a low or negative thermal coefficient of thermal expansion, and good thermal conduction properties, but also is compatible with very high frequency electronics. The present invention satisfies this need.